Slide 1
Do traits on sex chromosomes behave differently? Why or why
not?
Slide 2
Yes and no - yes because males and females have different sex
chromosomes (males=XY, females=XX). No because sex chromosomes go
through meiosis just like autosomes.
Slide 3
Humans (and most mammals) as well as fruit flies have an X-Y
system of sex determination - females have two copies of the X chromosome and
males have an X and a Y.
During meiosis, the X and Y chromosome behave somewhat like a homologous pair - in males the X and Y segregate from each other just like homolgous pairs do - the result is that males produce two kinds of gametes - gametes with an X chromosome or gametes with a Y chromosome.
Females can only make gametes with an X chromosome.
Slide 4
Sex-linked genes are genes that are found on the
sex-chromosomes. Because males and females have a different complement
of sex chromosomes we expect sex-linked genes to show a different pattern of
inheritance in males and females.
For the purposes of this course, we will limit our discussion of sex-linked genes to genes found on this X chromosome. This is because the Y chromosome is significantly smaller than the X chromosome and does not contain many genes (although the Y chromosome does contain the genes that make a male a male).
Even though the X and Y behave like homologs during meiosis - there is in reality very little homology between them - in other words, they do not have the same genes.
This means that typically males only have one copy of each gene found on the X chromosome. We use the term "hemizygous" to describe this condition. A male cannot be homozygous or heterozygous for X-linked genes - because he only has one copy of the X chromosome.
Slide 5
The X-Y system of sex-determination is only one way sex is
determined in organisms. Some insects, for example cockroaches, use a
system known as X-O - females have two X chromosomes just as in humans but
males just have a single X chromosome.
Birds have a system known as Z-W - there are two different sex chromosomes but in birds it is the female that has two different sex chromosomes and the male has two that are the same.
Some insects, known as the eusocial insects because of their highly social behavior and include species of ants and bees, have a sex-determination system known as haplo-diploidy. In these insects there are no distinct sex chromosomes - instead the males are haploid and the females are diploid.
Slide 6
The specific examples of sex-linked inheritance that we will look
at will only involve organisms with an XY system of sex determination such as
humans and fruit flies.
There are some important consequences of gemes being found on the X chromosome that are not found on the Y chromosome.
For example, only females can be carriers for sex-linked recessive traits - remember that carriers are heterozygous and males cannot be heterozygous for X-linked traits (and are termed hemizygous).
Therefore, males will always express an X-linked trait - whether it is dominant or recessive. This is in contrast to females - for a female to express a sex-linked recessive trait she must have two copies of the recessive allele.
Slide 7
Here are some examples of Punnett squares that examine sex-linked
crosses. Notice that when creating a Punnett square for an x-linked
trait you must include both the X and Y chromosome for the male so you can
keep track of inheritance differences between males and females.
In scenario 1, a homozygous dominant female is crossed with an affected male (this trait is x-linked recessive). The result is that all female offspring will be carriers for the trait and all males will be unaffected.
In scenario 2, the female is a carrier and the male is unaffected. In the resulting offspring, the females are either homozygous dominant or carriers. The males are either unaffected or affected.
In scenario 3, the female is a carrier and the male is affected. This is the only scenario in which it is possible to see affected female offspring.
X-linked recessive traits are expressed more commonly in males than in females.